Myeloid derived suppressor cells (MDSC) are a heterogeneous, ill-defined population of immune regulatory cells that are a critical cell population in suppressing tumor immunity and controlling inflammatory processes. Currently, MDSC function is based on studies of phenotypically defined cell populations that contain both suppressive and non-suppressive cells. Markers clearly defining functional MDSC are not available nor have functional pathways unique to MDSC been described. We recently showed that spleen Gr1+CD11b+ cells in mice, the prototypical MDSC phenotype, are precursor MDSC and that active regulatory MDSC are present only within a tumor or at the site of inflammation. Only tumor- or inflammation-derived MDSC to express Arginase I (Arg-I) and inducible nitric oxide synthase (iNOS) and suppress T cell responses, clearly localizing functional MDSC to active inflammatory sites. Thus, array analyses on MDSC both from spleen and tumor were performed to identify gene and micro-RNA (miRNA) expression patterns restricted to tumor-derived (functional) MDSC. Gene array analysis showed 150 genes to be upregulated at least 5-fold, including Arg-I (129-fold) and iNOS (33-fold), in tumor-derived (functional) MDSC compared to spleen (precursor) MDSC. Among the novel genes differentially expressed, to date two, slc7a2 and vitamin D receptor, were observed to regulate MDSC T cell suppressor activity. Thus we hypothesize that identifying miRNAs linked to MDSC function will identify new targets that will enable identification and modulation of active MDSC. Additionally, integrating the miRNA analysis with the gene array data will enhance pathway discovery unique to functional MDSC. Differential miRNA arrays were performed on the same spleen- and tumor-derived MDSC that were used for the gene array analysis. The miRNA arrays show a statistically significant 2-fold or greater up-regulation of 46 miRNAs and similarly a 2-fold or more down- regulation of 40 miRNAs. We propose to utilize the differential miRNA arrays in conjunction with the gene arrays to identify novel MDSC-specific genes and pathways that are associated with MDSC function. The identification of regulatory miRNAs and associated pathways will provide the foundation for better understanding the molecular basis for MDSC function and also will provide opportunity to better target MDSC and/or identify the presence of active MDSC. Two specific aims are proposed: (1) Identify genes, gene sets, pathways, and network modules that are significantly related to differentiating between functional and precursor MDSC to gain global insights of the biological process and its relevance to cancer. Hypothesis: functional MDSC utilize distinct functional pathways from precursor MDSC that will distinguish the two populations. The differential pathway expression linked to function can be detected at the systems level through pathway/network modeling and integrative expression profiling and (2) Validate the biological relevance of select miRNA identified via the analyses performed in Aim 1.